The present invention is in the field of composite materials exhibiting improved fracture toughness and strength at high temperatures, and particularly relates to improved composite ceramic materials useful for high-temperature applications such as the high-pressure extrusion of ferrous or non-ferrous metals.
U.S. Pat. No. 3,365,317 describes metal extrusion dies exhibiting lubricity, toughness, abrasion resistance and chemical inertness formed of a magnesia stabilized zirconia composition. Such dies are intended for use at high pressures and at temperatures which may range as high as 1650.degree.-2370.degree. F.
Zirconia extrusion dies of the type described in the aforementioned patent have been commercially available for some time. For example an MgO-containing partially stabilized zirconia material commercially available as Corning Code 1027 zirconia has been utilized for many years to fabricate extrusion die nibs for the production of copper alloy rod.
Unfortunately, the partially stabilized zirconia materials which had been utilized for extrusion applications prior to the present invention had exhibited considerable variability in the toughness and resistance to cracking failure which they exhibited in actual use as an extrusion die. In some cases, the failure of one die from a die lot might occur after only a few extrusion runs, while another die from that lot might exhibit unexpectedly prolonged life under identical extrusion conditions. This behavior could be attributed to variability in the crystal phase makeup of the extrusion dies and/or to a lack of consistent die casing and extrusion practices. The precise crystal forms taken by zirconia depend strongly on the amount of stabilizer used, and on the thermal history of the partially stabilized zirconia product. A variety of crystal assemblages can be produced in a single nominally invariant composition using different firing schedules.
Attempts have recently been made to improve the performance of zirconia products for extrusion die applications by enhancing the toughness and/or strength of the material. Such attempts have included processing modifications such as thermal aging procedures by which the strength and toughness of zirconia products can be enhanced. However, such processing adds to the expense of the product.
A number of other ceramic materials are known which exhibit good high temperature stability and strength. Examples of such materials include the class of materials known as glass-ceramics, which are largely crystalline bodies produced by the controlled crystallization of glasses of appropriate compositions. U.S. Pat. No. 2,920,971 describes some early glass ceramic compositions exhibiting good high-temperature properties, and many other glass-ceramic compositions have been subsequently developed for various applications. Unfortunately, because of the presence of residual glassy phases therein, such materials are not expected to exhibit the high-temperature creep resistance necessary for metal extrusion applications. Such applications customarily demand die shape retention to maintain high dimensional tolerances on the extruded product.
Families of ceramic materials have also been considered for use in high temperature applications wherein toughness is required. In general, however, ceramic matrix materials entirely devoid of residual glassy phases do not exhibit high inherent toughness. Rather, these wholly crystalline assemblages are prone to cracking fatigue and breakage and provide a material wherein crack propagation e.g., between grains of the crystalline material is relatively easy. Thus it has been proposed to strengthen ceramic bodies with inorganic whiskers or fibers to enhance the strength and toughness thereof.
The use of strengthening strategies involving reinforcement by inorganic whiskers and fibers has been discussed by N. Claussen in "Strengthening Strategies for ZrO.sub.2 -toughened Ceramics at High Temperatures", Materials Science and Engineering, 71 (1985), pages 23-38. That article proposes that a number of whisker-toughened materials including cordierite, mullite, and zirconia would be useful subjects for exploratory studies in the area of reinforced ceramic composite materials.
In U.S. Pat. No. 4,464,192, products formed by molding processes are described which include a glass ceramic matrix composed of a lithium aluminosilicate, magnesium aluminosilicate or similar glass-ceramic material and a reinforcing chopped fiber or whisker phase composed of silicon carbide or the like. These materials are proposed for use, for example, as engine parts for high-temperature use.
In the copending commonly assigned patent application of K. C. Chyung et al., Ser. No. 723,572 filed Apr. 15, 1985, now U.S. Pat. No. 4,615,987, a family of whisker-reinforced alkaline earth aluminosilicate glass-ceramic materials comprising 10-60% by weight of a silicon carbide fiber or whisker reinforcement is described. These materials, which may contain principal crystal phases such as anorthite combined with mullite, cordierite, albite and/or barium osumilite, reportedly exhibit improved toughness (resistance to cracking) and good high temperature strength. However, reductions in strength and toughness were observed even in these materials at temperatures of 900.degree. C. Moreover, the retention of glassy phases in these largely glass ceramic materials would be expected to significantly limit the high temperature creep resistance thereof.
Therefore, notwithstanding the various proposals for toughening glass and ceramic products, no commercially successful reinforced composite ceramic product has yet been developed for such high-temperature applications as die parts for metal extrusion processes or the like.
It is therefore a principal object of the present invention to provide a composition system particularly well suited for the fabrication of stress parts useful for high temperature applications. It is a further object of the invention to provide extrusion dies or similar components for metal forming composed of composite glass-ceramic materials exhibiting both excellent creep resistance and enhanced strength and toughness and to provide a metal extrusion process employing such dies.
Other objects and advantages of the invention will become apparent from the following description thereof.